Seconding (thirding? Fourthing? Eh, who knows) the exposure argument, about how a lot of traditional Christian fiction (or, really, just fiction written by Christians) leans more towards fantasy than sci-fi. If Tolkien had created an inticrate alien society with a galaxy far away from ours and Lucy Pevensie had stumbled into a world populated by AI, maybe the landscape would look different today. But a lot of staples in Christian literature (C.S. Lewis (though Out of a Silent Planet is an exception), J.R.R. Tolkien, G.K. Chesterton, etc.) are fantasy, and I think a lot of us stick with what we’re familiar with.
That said, I believe the sci-fi market might be even more in need of Christian-written books than fantasy, and that the fact that few Christian writers (in comparison to fantasy) have chosen to go that route shouldn’t keep us from doing it but maybe even push us to doing it – because no one else is.
The tale is a lie; what it tells is the truth. - Traditional folk tale ending